This invention generally relates to an improved method and apparatus for determining the weldability of metals and more particularly to a crack susceptability test especially designed for metals having a modulus of elasticity less than steel, such as titanium, aluminum and copper based alloys.
A critical factor affecting the suitability of base-filler metal combinations for various constructions is the development of cracks in the weld metal. Weldment cracking is affected by the interaction of many complex factors, such as, for example, the particular base-filler metal combination; the mechanical restraint provided by the joint structure; and the presence of low fracture toughness and high yield strength in the weld zone. Also, some materials such as titanium alloys present additional difficulties because of their high reactivity with elements such as hydrogen, oxygen, and nitrogen and because of weld embrittlement accompanying contamination by these elements.
Cracks form during the welding cycle when the strains set up by localized residual stresses and the longer range reaction stresses exceed the critical strain to initiate fracture in the weld zone. During welding, the residual strains in the weld are continuously changing as a function, for example, of localized temperatures; the overall temperature distribution in the weld zone; differential cooling and heating of the weld zone; and the sequence of metal deposition. Thus, the weld zone has a continuously changing potential for fracture and if the applied strain coincides with or exceeds the critical strain at any time or location, a crack can form.
Although the complex interaction of various factors affecting weld cracking make it difficult to predict the weldability of an alloy, several approaches have been developed to determine the suitability of a weld-filler metal combination. One approach involves the extrapolation of the results of small scale mechanical tests to predict the behavior of actual, large scale welded structures. Although the applied stresses in these tests can be accurately controlled and determined, the results often do not accurately represent the complex stress distribution in the, weld zone.
Another approach to determine the suitability of a weld-filler metal combination involves making self restrained welds under conditions which simulate the restraint achieved in actual production welds. In general, the degree of restraint can be altered by changing the dimensions and rigidity of the restraint specimen, or by using a different type of restraint specimens. Examples of crack susceptibility or crack restraint specimens are disclosed in pages 32-45 of DMIC report 244; August 1968; Weldment Evaluation Methods; by the Defense Metals Information Center, Battelle Memorial Institute, Columbus, Ohio.
Another example of a crack susceptibility test is disclosed in U.S. Pat. No. 3,526,948. However, crack susceptibility tests utilizing some of the tests embodied in the above-mentioned disclosures to determine the crack susceptibility of metals having a modulus of elasticity less than steel, such as titanium alloys, have failed to provide the restraint necessary to generate cracks in the weld zone.